The invention relates to torque wrenches and more particularly to a light-weight, portable torque wrench wherein the torquing force is supplied by fluid power cylinders arranged such that equal, and diametrically opposed forces are applied to the part being torqued without induced side loading.
Many current applications require high torquing forces to be applied with great precision, while simultaneously maintaining a precision alignment of the parts being joined. This is true, for example, in the assembly and repair of jet engines and other high speed machinery. For example, the removal or installation of a main rotor shaft may require torques exceeding 6000 fb.lbs. while maintaining tolerances of less than 0.001 inches. It has been found that prior art torque wrenches, such as those discussed below, can not always meet these requirements. A typical prior art design utilizes an offset, single mechanical moment arm or single hydraulic ram assembly which has the tendency to cause side loading, especially at high torques. Such side loading can lead to bending or eccentricity of the parts being joined. Even a slight eccentricity, for example, in a turbine shaft, can cause unacceptable run-out with an attendant increase in engine vibration, leading to premature engine failure.
In the field of torque wrenches there is a wide selection of known wrenches designed for particular applications. Even in the somewhat narrower field of fluid power torque wrenches there is a great variety of designs. U.S. Pat. No. 4,137,800 to Austin, for example, discloses a torque wrench in which a hydraulic cylinder forces a slide block against a single torque arm causing the rotation of a ratchet mechanism. U.S. Pat. No. 3,686,983 to Flagge discloses a torque-applying device wherein the torquing force is supplied by a hydraulic motor. One example of a specialized torque wrench available in the prior art is disclosed in U.S. Pat. No. 4,838,130 to Snyder which describes a hydraulically actuated power wrench specifically adapted for torquing one of a plurality of adjacently spaced bolts. The Snyder '130 wrench comprises a hydraulic cylinder which acts on a pivotable lever to turn a bolt. The reaction force due to the application of torque is borne by an adjacent bolt through a specially shaped anchoring ring. U.S. Pat. Nos. 3,868,872 and 4,706,527 illustrate further examples of torque wrenches where the torquing force is provided by a hydraulic cylinder acting through a lever arm.
U.S. Pat. Nos. 3,625,095 to Barnet et al. and 4,398,598 to Fabrygel and also USSR patent No. 747,709 each disclose a torquing device that applies a torquing force through a single opposed pair of linear forces. While it may appear that a single opposed pair of linear forces (a force couple) would theoretically provide a balanced torquing force, in reality it does not do so. This is due to the high torque and the practical problems encountered in actual manufacture and operation of the prior art wrenches. These prior art devices utilize a hollow cylinder or ring for converting linear force to angular torquing force. During torquing, and especially at high break-away and torque-up torques, the hollow cylinder or ring will behave as if it were essentially fixed against rotation due to the reaction forces created in the part being torqued.
In general, if a single pair of parallel and opposed forces is applied along the circumference of a fixed cylinder or ring, as in the prior art devices, the circumference of the cylinder or ring will tend to become elliptical if the forces are great enough. If the cylinder is provided with extending legs, to facilitate the application of forces, the elliptical effect is increased due to the bending moment created at the interface of the leg and cylinder. Depending on the means by which torque is transmitted from the wrench to the article being torqued, this deformation can also be transmitted to the article.
Generally, the use of heavy bearings to compensate for the elliptical effect has been the attempted solution in the prior art for high torque applications. However, there are at least two significant disadvantages to such construction: First, deformation will still occur in the ring or cylinder to the degree allowed by the bearing tolerances. Also, the load carried by the bearings creates frictional losses in the bearings, which result in inaccuracies in torque measurement.
The two disadvantages of prior art wrenches discussed above are present even if all parts of the device are perfectly aligned, which in actual practice rarely occurs. In actual practice, eccentricities are present due to machining tolerances in both the wrench and the article being torqued. Further eccentricities may be introduced by slight misalignment or out-of-roundness of the article being torqued. These eccentricities will cause a single opposed pair of torquing forces to act with unequal moment arms, thereby applying unequal torques. This unequal application causes the elliptical effect to be exaggerated where the moment arm is longer, causing an egg shape as opposed to a symmetrical ellipse. The creation of an egg shape means that unequal forces are acting against the supporting bearings. Thus, a resultant force is created which can be transmitted to the article being torqued and cause significant damage to that article. Such a resultant force is referred to as "induced side loading".
U.S. Pat. No. 2,961,904 discloses a hydraulically actuated wrench which attempts to address the failings of the prior art by applying a three-point balanced torquing force. The '904 wrench is provided with a central drive member having a ratcheted opening for engaging a nut. The drive member also has three arms equally spaced at 120 degree intervals extending radially from the drive member. Three pairs of fixed, opposed hydraulic cylinders act on these arms to provide the torquing force. The cylinders are equally spaced about the drive member in order to exert a balanced force on the work piece. The cylinders are arranged in pairs in order to enable the wrench to operate in either direction, i.e., three cylinders exert a clockwise force and three exert a counter-clockwise force.
While the patented wrench disclosed in the '904 patent is an improvement over the previously available wrenches in that it applies a generally balanced torquing force, its design is still not satisfactory for all applications. The triangular arrangement of the cylinders, while generally providing balanced forces, is inherently unstable due to the fact that each cylinder, acting on the work piece through the drive member, is not directly opposed by an equal reaction force. Therefore, the possibility exists for induced side loading to occur, causing the piston to creep on the arm of the drive member. Such creep would change the effective length of the moment arm and thus cause inaccurate torque readings.
Another disadvantage of a design using opposed cylinders for supplying torquing forces in two directions is the precision machining which is required to fabricate such a wrench. In order for the torque readings to be accurate in both directions, the center lines of the opposed cylinders must match exactly. If the center lines do not match, there will be different effective moment arms depending on which cylinder is acting. It is expensive and time consuming to precision line bore the required seat for the opposed cylinders. In large wrenches, requiring large diameter bores, the tolerance requirements alone may be sufficient to cause a center line mismatch.
In addition, paired cylinders such as are found in the '904 wrench require two travel limit valves to prevent piston over travel. This extra hydraulic valving must be placed on the outside of the wrench as a result of design size considerations. The valving is therefore subject to damage and leaking when the wrench is used under normal field conditions.
A further disadvantage of the prior art wrenches discussed above is that in order to achieve a large torque capacity, the weight and size of these wrenches are such that they are bulky and cumbersome to use. This is especially true of the wrench of the '904 patent because six cylinders are included while only three at a time are used for a particular torquing operation. This greatly increases the weight of the wrench. Portability is a very important feature in torque wrenches as described herein, especially if the wrench is to be successfully utilized at remote field installations as is required in the offshore oil industry and in many military applications.